Jiming Yang,  The Error Analysis of An Upwind Difference Approximation for a Singularly Perturbed Problem.   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    April 2011, vol. 52(4). 621 - 623
    [viewed 19 January 2022]. Available from: https://publications.waset.org/pdf/9939.